This invention relates to trunk communications networks for computers and more specifically to trunk installations comprising local area networks (LANs).
The criteria involved in coupling fixed computer work stations into a main frame installation have been established over a long period. More recently, the advent of personal computers has led to the requirement for a trunking arrangement for multi-station communications with the facility of being able to withdraw any of the work stations from the network system by means of a simple plug and socket arrangement and to re-insert a station into the network at a different location, with the network being and remaining transparent to the computer of the work station and any computer communicating therewith. This concept has an analogy in the concept of the a.c. power ring main for domestic dwellings whereby power outlet sockets are installed in almost every room. As with the domestic ring main there are laid down standards, though because the field of the present invention is one which is rapidly and continuously evolving, many of the relevant Standards, including those pertaining to the field of the present subject matter, are still in draft form and not officially adopted. Nevertheless these Standards, or Drafts, as the case may be, have international recognition.
The above-described analogy does not provide any indication of the complex requirements of a computer trunk system, Computers cannot be inserted into a trunk system and communicate thereon in a random, disorganised manner, nor may they be inserted without regard to the integrity of the system as a whole. Thus when a work station is not contributing to any communication exchange through a trunk system or when the computer is disconnected from the trunk system the latter must remain available for instant use or re-use in all of its parts. Hence, trunk systems are protected by specific laid-down procedures which must be undergone when any computer is inserted into the trunk. These procedures are known as protocols. Protocols exist at several levels of the communication process and involve both hardware and software. On insertion of a work station into the trunk system, if the installation protocol for that work station is interrupted due to any cause, then the connection is not made, the trunk system automatically bypasses the work station, and its integrity is thereby preserved. Thus at the physical level, involving the insertion of a work station "plug" into the trunk "socket", a number of electrical tests are performed in sequence and only upon their successful conclusion is the work station actually electrically connected onto the trunk system, this being done by means of relay contacts in the socket the relay of which is responsive to a signal from the work station once the requirements of the protocol have been satisfied. When the work station is connected to the trunk system it becomes, so to speak, an integral part thereof.
By virtue of the afore-mentioned need of the trunk system to be carefully protected, that the latter takes on the aspect of a permanently fixed installation, again like a power ring main, and like any such installation, such an installation is not easily adaptable to changing needs with regard to the location and concentration of work stations. Indeed, in the case of computer work stations adaptation has to be shunned wherever possible to preserve the aforesaid integrity of the system.
Several trunking system topologies are known, the names of which are mostly self-explanatory, such as a star network configuration, master/slave network configuration, etc. The present invention relates most closely to, but is not of, species of local area networks (LANs) known as the token ring network topology. In a token ring topology the trunk itself comprises a normally closed ring around which are a number of entry points at which, by means of relay switching, work stations may be inserted into the system. Any computer can communicate with any other but by virtue of the aforesaid protocols, transmission takes place in packages and only one computer is allowed the use of the ring at any one time to transmit one such package and is regarded as the computer having, at that time, the so-called "token". When the package has been transmitted that computer relinquishes its monopoly of the ring and the token passes to another computer on the ring.
The token ring network is an example of the broader class of networks which are defined in the relevant Standards as "local area networks" (LANs) and, as the name suggests, they are particularly configured for use within a limited and confined circumference of such form whereby communication may take place directly between one computer and another without any need for using modems, etc. These LANs are covered by a family of I.E.E.E. Standards called IEEE.802 and there are corresponding ISO and BSI documents, though these are still in the draft stage. The subject of the "token ring" local area network is specifically covered by IEEE.802.5 and the corresponding International Standard Number is 8802.5, reference ISOTC 97 SC 6N 3244. The BSI document reference number is DD 136:1986.
The token ring local area network, as conceived and standardised, though catering for a plurality of work stations, is not adapted to cater easily for changing needs in the location of work stations, as may occur upon the relocation of office staff.